This invention relates to position control systems and methods for translating one member relative to another; and, more particularly, this invention relates to methods and apparatus for moving a memory data device member such as a data transducer relative to another member such as to a desired concentric track of a rotating disk, and keeping the moved member in desired alignment with the other member, e.g. the transducer in registration with the track.
In electromechanical devices such as the rigid rotating memory devices disclosed by the prior art, two basic approaches have been taken in order to position data transducers radially relative to the rotating magnetic disk surface. A first, high cost approach was to utilize a dedicated servo system with a servo head and a replicated servo surface on the disk to provide a high performance track-following transducer positioning scheme. On the other hand a more recent, low cost approach was to utilize completely open loop stepping motor positioners which operated to place the transducer at arbitrarily defined tracks without any actual position information being fed back from the disk to the head positioner. The main drawback of the prior art low cost open loop approach was the requirement that tracks be spaced far enough apart to take into account all of the variations of the system, including mechanical tolerances in the stepping motor actuator, thermal expansion of the disk, and disk run-out. The result was a disk drive product which, while effective as a low cost unit, lacked the data storage capacity of the more expensive units with the result that the cost of storage per bit stored of the low cost drives approached the same cost as the earlier, more expensive storage units